In recent years, a demand for “digitization of X-ray image information” has increasingly arisen in the medical field. If such digitization is achieved, a doctor can examine X-ray image information of a patient at an optimal angle in real time, and the obtained X-ray image information can be recorded and managed using a medium such as a magnetooptical disk or the like. Using a facsimile or other communication methods, X-ray image information of a patient can e sent to any hospitals on the globe within a short period of time.
In nondestructive inspection represented by inspection of the interior of an object such as the skeleton of a building or the like, installation of various devices required for X-ray image sensing, and image sensing of a required portion cannot be done so frequently.
Therefore, in such field, a demand for providing X-ray image information of a desired portion in real time is increasing. Hence, an X-ray image sensing apparatus which uses a CCD solid-state image sensing element or amorphous silicon sensor in place of a film has been proposed recently.
An example of a radiation image sensing apparatus that we proposed previously will be explained below.
FIG. 11 is a circuit diagram showing the arrangement of a two-dimensional (2D) area sensor. FIGS. 12A and 12B are respectively a plan view and sectional view of respective building components corresponding one pixel of the 2D area sensor; FIG. 12A is a plan view and FIG. 12B is a sectional view.
The radiation image sensing apparatus shown in FIG. 11 comprises a 2D matrix of a total of 16 pixels 1103, i.e., 4 cells in the vertical direction×4 cells in the horizontal direction. Each pixel 1103 comprises a set of a sensor element 1101, and a transfer transistor 1102 connected to the element 1101.
The sensor elements 1101 are connected to a bias means 1104, and the gates of the transfer transistors 1102 are connected to a shift register 1105 via gate lines. Output signals of the transfer transistors 1102 are transferred to an amplifier/multiplexer/A/D converter 1106 via signal output lines, and are processed in turn. A reset means 1107 is connected to the signal output lines of the transfer transistors 1102.
A portion bounded by the broken line in FIG. 11 is formed on a single, large-area insulating substrate 1108. FIG. 12A is a plan view of a portion corresponding to one pixel.
As shown in FIG. 12A, a photoelectric conversion element 1101, TFT (thin film transistor) 1102, and a signal line SIG are formed. FIG. 12B is a sectional view taken along a broken line A–B in FIG. 12A.
According to the layer structure shown in FIG. 12B, the photoelectric conversion element 1101, TFT 1102, and signal line SIG are simultaneously stacked and formed on an insulating substrate 1. These components are formed by only stacking a common lower metal layer 2, silicon nitride layer (SiN) 7, i-layer 4, n-layer 5, and upper metal layer 6 in turn on the insulating substrate 1, and etching these layers. After that, a P-layer 23, I-layer 24, and N-layer 25 are formed as the photoelectric conversion element 1101, and an upper electrode layer 26 of ITO or the like is formed on the element 1101.
Also, a passivation silicon nitride film (SiN) 8 and a phosphor 12 which is made up of CsI, Gd2O2S, or the like and wavelength-converts radiation into visible light, are formed on the upper portion of a pixel. When X-rays 13 that contain image information enter the radiation image sensing apparatus, they are converted by the phosphor 12 into image information light 14, which enters the photoelectric conversion element 1101.
An X-ray automatic exposure controller (AEC) that automatically controls the exposure of X-rays emitted by an X-ray source in the radiation image sensing apparatus will be explained below.
In general, in the radiation image sensing apparatus having a 2D sensor matrix, the amount of incoming light must be adjusted (undergo AEC control). This control can be classified into the following two processes.
(1) An AEC control sensor is arranged independently of the radiation image sensing apparatus.
(2) All or some sensor elements in the radiation image sensing apparatus are read at high speed, and are used as an AEC control signal.
Conventionally, a plurality of low-profile AEC control sensors with an X-ray attenuation ratio of around 5% are arranged on the front surface of a 2D sensor that converts an incoming X-ray pattern into a 2D image, and X-ray radiation is stopped by the outputs from these AEC control sensors, thus obtaining an X-ray dose suited to image formation. As an AEC control sensor used in this case, a sensor that directly extracts a charge using an ion chamber, or a sensor which externally extracts phosphor light via a fiber, and converts the extracted light into a charge using a photomultiplier, is used.
However, when the AEC control sensors are independently provided to the radiation image sensing apparatus having the 2D sensor matrix so as to adjust (AEC control) the amount of incoming light or dose, the layout of the AEC control sensors poses a problem. That is, information required for AEC control is present at the central portion of an object, and in order to lay out the AEC control sensors without disturbing image sensing of the image sensing sensor, another optical means or AEC control sensors which have a very low optical attenuation ratio are required.
When all pixels are used, AEC control can be done in a sensor with a relatively small number of pixels. However, in a sensor with 2,000×2,000 pixels or more, a high-speed driving circuit is required, resulting in an increase in cost of the overall apparatus.
Since high-speed driving is required, it becomes difficult for the sensor of the radiation image sensing apparatus to assure a sufficiently long charge accumulation time, charge transfer time, and capacitor reset time, and the like, resulting in poor image quality of a sensed image.